1. Field of the Invention
The present invention generally relates to conduits for utilities such as pipes for natural gas, power and water distribution, as well as joints, valves and closures for the environmental protection of cables, such as those used in the telecommunications industry, whether for copper or fiber optic cable, and more particularly to novel splice closure and conduit designs having improved resistance to damage caused by certain animal pests, such as rodents and birds.
2. Description of the Prior Art
Damage from animals, particularly rodents, has plagued telephone and utility plant delivery systems since the early 50's, indeed, since the advent of conduits formed of polymers such as polyethylene (as used herein, "conduit" generally refers to pipe, duct or cable, and joints, valves and closures therefor). Squirrels, gophers, rats and mice will chew cables to grind their teeth down; closures designed for protecting drops, splices, etc., often suffer a similar fate. Gopher incisors will grow up to 12 inches per year and are thus required to be ground down for the animal's survival. These animals can exert a biting pressure exceeding 18,000 psi. Aerial closures have also been attacked by birds, particularly woodpeckers. When cables or pipes become so damaged, various deleterious effects occur. In the case of a copper communications cables, water ingress is common and typically leads to high resistance faults to the wires resulting in noise on the phone lines. Damaged pipes can rupture and leak resulting in some cases in fire, asphyxiation and explosion hazards. Animal damage seems unpredictable in its targets and extent, and utility companies have spent millions of dollars in the repair of damaged lines.
Several approaches have been devised to reduce rodent damage. One involves the design of the shape and size of the cable or closure, such as utilizing a circular cross-section with as large a diameter as possible, to prevent the animal from getting its mouth completely around the conduit, or otherwise present a larger radius of curvature to the geometry of the opposing teeth of the upper and lower jaws of the animal, so that the cutting force is more of a glancing blow. The clear disadvantage to such a requirement is the corresponding prohibition against small-diameter or irregularly-shaped conduits and closures.
Another parameter of animal resistance is the thickness of the outer wall of the conduit. While increasing the thickness is relatively simpler, it also leads to increased materials cost and increased weight of the pipe, cable or closure, without really making it any more impervious, since increased thickness neither prevents damage nor deters the animal, but rather simply makes it take longer to chew or peck through.
A third parameter recognized in the art for minimizing animal damage is the hardness of the outer surface of the conduit. Increasing the hardness generally does not, however, provide sufficient protection where the cable jacket or closure body is polymeric, since the polymers are typically selected because of their moldability or extrudability, and consequently cannot achieve sufficiently high hardness values to really prevent animal damage.
A fourth approach is to place a barrier layer around the conduit, such as a steel cylinder, guard, cover or tent structure. See, e.g., U.S. Pat. No. 4,262,169, and "Animal Chew Protection for Cables and Devices," ISA '88 Conference pp. 1823-1834. One significant drawback to this approach is the use of metal in the barrier. The presence of metal, particularly along any significant length of the conduit, may result in extensive damage associated with anodic corrosion, chemical corrosion, and lightning strikes, and utilities accordingly try to avoid the use of metal barriers. Non-metallic materials may be substituted, but they are generally less effective due to their reduced hardness. For example, the article "Protecting Cable From Rodent Chew," Outside Plant, November/December 1988, describes a fiberglass/urethane fabric which may be wrapped around the cable. While the urethane resin is somewhat resistant to impact and bending fractures, it certainly does not provide the kind of protection imparted by metallic barriers. Some authors explicitly note that nothing less than stainless steel will prevent barrier penetration, while other commentators opine that even stainless steel and other hard alloys will ultimately fail due to deformation wear. Another variation of the barrier approach is the use of a "cage" or wire screen, made of a hard metal, which prevents the rodent's teeth from reaching the outer polymeric layer of the cable/closure; see, e.g., "Pocket Gophers Gnawing Electric Cables," J. of Wildlife Management, vol. 17, no. 3, pp. 296-298. This design generally suffers from the same limitations as solid steel armor.
The barrier may alternatively take the form of an intermediate layer of the conduit, such as a metallic sheath which is usually found inside a cable's outer jacket; see, e.g., Lightguide Digest, 1992 Issue no. 2, pp. 1-3. Such a shield presents the same problems, however, associated with metallic components. Non-metallic compositions have been formulated for intermediate layers, as described in U.S. Pat. Nos. 4,874,219, 4,938,560 and 4,946,237, but these similarly lack the strength (hardness) of steel shielding. All of these barriers (metallic and non-metallic, exterior or interior) also add appreciably to the cost of the cable or closure, especially when considering that there is a very large area that must be covered on the cable/closure, and often increase the difficulty of installation. A further disadvantage in the use of cables or closures having intermediate protective layers is that, when the outer plastic layer is removed by gnawing or pecking, although the inner layer may temporarily halt animal damage, it leaves the inner layer exposed to the environment which eventually leads to the corrosion of the inner layer. This not only makes it easier for animals to subsequently inflict additional damage, but moreover may lead to a serious fault in the cable even if there is no further animal activity.
A commonality in each of the foregoing implementations is that these constructions are designed to provide physical protection (i.e., impact resistance) as opposed to deterring the animals from chewing at all. A deterrent approach is found in U.S. Pat. No. 4,309,072, which discloses the use of an outer layer having a plurality of spikes intended to discourage the animal from attacking the cable in the first place. This approach is defective, however, because its feasibility is limited to the use of spikes formed from a polymeric material and, as noted in the ISA '88 article (at page 1825), "polymer spike devices applied over cable only temporarily delay rather than halt damage to small cable such as optical fiber."
A combined protection/deterrent approach is found in U.S. Pat. No. 4,505,541, in which the cable has an inner layer of resin-impregnated inorganic fibers which, when bitten, releases needle-like projections designed to irritate the animals. This approach is also defective, however, since they can avoid getting any of the material in their mouth. As noted in the Outside Plant article, rodents have a space between the large incisors and the cheek teeth. This gap allows them to gnaw without having cuttings of the material enter their mouths, and they will not open their mouths while chewing except to get what they want and previously know as food. Moreover, the Wildlife Management article cited above notes that gophers may actually desire such filamentous materials in order to line their nests. Bitter tasting chemicals have also been applied to the conduit, but the Wildlife Management article concludes that poisons and repellents do not work against rodents because again the chemical does not get in animal's mouth. It would, therefore, be desirable to devise a method of protecting conduits, etc., against rodent chewing damage, which method not only provides a physical barrier but further provides a deterrent effect that does not rely on the animal ingesting any substance or getting some material on its coat. It would further be advantageous to devise such a method that is easily practiced, not only in the original fabrication of the article, but also in the repair of existing conduits, closures, etc., and does not require metal.